Testing

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Metal removal fluids need to be checked often to make sure that the fluid is performing as it should. MRF must be kept in good condition. That means keeping it free of contaminants, keeping fluid components at the proper levels, and testing for various chemical and physical properties.

Testing metal removal fluids

Some chemical tests of fluid condition can be done only in a laboratory using special instruments and apparatus, but some tests can be done in the shop with relatively inexpensive equipment. Dissolved oxygen, pH, and conductivity can all be measured by instruments that will fit in a shirt pocket; these are available from laboratory supply houses. These require frequent calibration, but the procedures are simple and the standards are inexpensive and reliable.

types of tests

concentration
alkalinity
conductivity
pH
oil concentration
emulsion stability
appearance
specific ingredients
salts
dirt
microbes

how often to test

obtaining
samples

concentration
checks:

acid split
refractive index
chemical titration

acid split

refractive index

chemical
titration

chemical and physical properties tested for

alkalinity

conductivity

oil concentration

emulsion
stability

appearance

specific
ingredients tested for

contaminants

chemical
products

oil

non-process

salts

dirt

gravimetric
testing

centrifuge testing

microbes:
plate counts
dipsticks
dissolved oxygen
catalase levels

acceptable level of microbes

plate counts

dipsticks

dissolved oxygen

catalase levels

level of microbes

frequency of testing by sump size:
miniature
small
small-medium
medium
large

Many suppliers can also provide simple titration kits or test papers that measure emulsifier content or alkalinity in a solution. These kits can provide a quick check for strength (concentration) and condition.

What kinds of tests are performed?

It is usual to check for

fluid concentration

physical and/or chemical properties of the bulk fluid

specific ingredient concentrations

contamination levels

How are samples of the fluid obtained for testing?

MRF samples should be drawn and tested in the same manner each time, with samples taken at the same time of day when possible. Each system should have a sampling valve in an accessible area on the clean MRF supply header. The sampling valve should be opened and MRF purged for 30 seconds before the sample is taken. No hose should be attached to the valve. If the system has vacuum filtration, the vacuum should be recorded when the sample is taken (contaminant levels may be higher if the filter medium has just advanced or "indexed"). If the system has pressure filtration, the pressure drop across the filter should be recorded

Why are concentration checks performed?

The single most important fluid characteristic is concentration. At the proper dilution, the fluid contains adequate levels of biocides, corrosion inhibitors, cleaners, and lubricants to perform as expected. If the concentration is too weak, the fluid simply will not perform as it should. The fluid will go rancid, parts and machines will rust, and tool life will be poor. If the concentration is too strong, problems with foam and residue on machined parts can arise. Concentrations that are too strong can also create the potential for adverse health effects to exposed employees, such as dermatitis or respiratory effects.

How are concentration checks performed?

Concentration is commonly determined by measuring some easily determined fluid property and using the result to estimate the dilution factor for the fluid as a whole. The test may be as unsophisticated as measuring the refractive index; it may require simple lab equipment and chemicals (acid split); or it may require a titration involving volumetric devices, and chemical reagents and solvents may be used. All these tests give only gross estimates, and their results should be used with caution.

Acid split

This test is performed only for semisynthetic and soluble oil metal removal fluids. In a centrifuge/acid split, sulfuric acid is added with care (to avoid spattering) to the sample in order to destroy the emulsion, and both are placed in a Winthrop tube or Babcock bottle. The tube is spun in a low-speed centrifuge for 15 minutes. The oil will separate from the water phase and form a layer at the top of the tube. The concentration is determined by the ratio of oil to water. Note that any tramp oil will be read as metal removal fluid.

Refractive Index

Any chemical present in solution will cause a change in refractive index relative to water. Good correlations exist between refractive index and concentration in "fresh," uncontaminated fluids. As soon as any contaminant is introduced, the strength of this correlation is compromised. In general, the refractive index gives a measure of the maximum concentration of a fluid; the actual concentration may well be less. No other reliable information can be learned from this test.

Chemical Titration

The amounts of fluid components in a diluted fluid can be determined by volumetric titration. The fluid concentration can then be estimated by assuming that all components are present in the diluted mix in the same ratios as in the unused fluid. The method may be as simple as titration with acid to determine the alkalinity of the fluid. At the other extreme, the method may involve reaction of the anionic emulsifiers in a fluid with a known amount of a cationic polymer. These methods commonly involve extraction of the sample with an organic solvent which may be toxic and unsuitable for use by untrained personnel outside a laboratory.

All these tests can be subject to interferences by contaminants in the fluid. Ideally, more than one test will be used to check for concentration.

How often should concentration checks be performed?

Concentration checks should be performed daily—certainly not less than twice a week. The frequency depends on the size of the system sump and past history of the system’s performance.

What chemical and physical properties of the bulk fluid, besides concentration, are measured?

Such properties include

alkalinity

conductivity

pH

emulsion stability

physical appearance

The major advantage of these measurements is that they are easily done with minimal equipment. It is important to remember, however, that a single result may not indicate anything particularly useful about the fluid condition. The tests can be used as a way to confirm what another test has indicated, or to alert you to the possibility of a problem.

It is assumed in this fluid management guide that those who need to be familiar with testing procedures will be familiar with the terms used in the following discussions of the properties listed above.

Alkalinity

For fresh, uncontaminated fluids, the correlation between alkalinity and concentration is good. Alkalinity will increase as a fluid ages because the alkaline components of a fluid are less likely to be stripped out by tramp oil, water hardness and other contaminants.

Many contaminants are themselves alkaline. However, alkalinity is a valuable fluid property to follow. Sudden increases in alkalinity signal contamination. Divergence of alkalinity from the anionic concentration signals possible fluid instability. Once the behavior of a system is known, it is possible to use alkalinity as a signal that fluid stability problems are imminent. The levels of alkalinity can also indicate that cases of dermatitis in employees exposed to the MRF may be about to occur.

Conductivity

The information gained from measuring conductivity is similar to that from alkalinity, but conductivity reflects the presence of all ions, while alkalinity is sensitive only to materials that are alkaline. On the other hand, alkalinity responds to materials that are not ionic, so the two tests confirm and complement one another.

Conductivity increases with use, reflecting the buildup of salts from the evaporation of make-up water over time. The conductivity of a system will increase steadily until solubility limits are reached. Thereafter, the conductivity remains constant; inorganic ions are lost through precipitation, and organic ions may be salted out of the aqueous phase into the oil phase. Once this loss of ions is taking place, fluid performance degrades and the emulsion can become unstable.

Alkalinity and pH are strongly related, but are distinct properties. The pH of a fluid is the same for any size sample. The alkalinity is measured by the volume of acid needed to titrate the sample to a given pH value and is dependent on the sample size. A pH meter should be used. The pH of a working fluid in good condition will tend to be around 8.8-9.2. However, the fluid supplier should be contacted for the appropriate pH range. Some fluids are designed to operate outside the pH range of 8.8-9.2.

The usual cause of pH drops is microbial growth, with resulting formation of metabolic acids. Keeping pH levels at 8.8 or above is an effective means of controlling bacterial growth and microbial odors. (Exceptions are some fluids for non-ferrous metals designed to be used at lower pH levels.) Caustic (sodium or potassium hydroxide) or sodium borate can be added to raise system pH. System pH should be monitored at the same frequency as concentration.

Oil concentration

It is common to estimate the concentration of soluble oils by splitting the emulsion with sulfuric acid and separating the oil by centrifugation (see p.44). Obviously, tramp oil will interfere with this test.

This method can be used to estimate tramp oil levels by testing for concentration by an independent method and calculating the amount of oil expected from the fluid alone and subtracting this from the observed amount of oil. Erroneous tramp oil levels are calculated if contaminants bias the results of the independent test or if the emulsion is unstable. When the calculated tramp oil level is negative, there is a clear signal that the fluid is unstable or that contaminants are interfering with the independent concentration check.

Emulsion stability

Emulsion stability can be evaluated by allowing a sample to stand undisturbed overnight or by centrifuging and observing the amount of free oil or cream that floats to the top. A sample is spun on a low-speed centrifuge (3000 RPM) for 15 minutes. The tramp oil will separate at the top of the liquid, the cream will separate to form the next level, and the suspended solids will settle to the bottom. A translucent or watery looking layer at the lower part of the tube indicates a weak (unstable) emulsion. This procedure should be done daily until the results are consistent enough to reduce the frequency. Tests should be done a minimum of twice a week.

Once the emulsion has become unstable, it may be possible to add emulsifiers to restabilize the fluid. However, the performance of the fluid will be compromised and the problem may recur, especially if the root cause has not been corrected. Excessive centrifugation can split the emulsion.

Physical appearance

The physical appearance of a fluid can provide indications of product problems. Look for cream or free oil layers floating to the top of the fluid, changes in appearance from translucent to milky, formation of haze (indicates a problem with fluid stability) or gray to black color formation (indicates microbial growth).

For what specific ingredients is the fluid tested?

In many cases, it is possible to check for individual fluid components for which specific analytical tests have been developed. It is important to know that interference will be present when contaminants contain chemical components that are similar to those in the fluid. Also, wet chemical methods may require the use of toxic chemicals that are not appropriate for use in some metalworking plants. Instrumental methods may require the use of sophisticated and expensive equipment. Specific chemical tests can be done by the supplier’s facility, but the data may be several days old before it is available to the user. The general tests and their applicability are given in the Table.

Class	Titration	HPLC	Electrochemical	Spot Test
Anionic surfactant	X	X	X
Cationic surfactant	X	X
Nonionic surfactant	X	X	X
Carboxylic Acids		X
Biocides		X	X	X

What contaminants are looked for in a contamination check?

Contaminants may be chemical products, oils, dirt, or material not related to the metal removal process at all; they may also be microbial.

Chemical products, oils, dirt, and non-process contaminants

Many chemical products can be found in metalworking plants. It is best to assume that all of these can and do get into sumps and become part of the fluid. These include

hydraulic oils
spindle oils
way lubes
gear lubes
greases
rust preventatives
parts cleaners
floor cleaners

Oil that gets into the metal removal fluid is called tramp oil and should be removed by skimming, coalescence filters, or centrifugation. See oil concentration.

Non-process contaminants—everything from bacteria, mold, and yeasts, to food scraps, cigarette butts, and shop rags—also find their way into the MRF system.

Salts accumulate in fluids through evaporation of water or by being carried in on work pieces. At high ionic strengths, emulsions are less stable, and the fluid performance will decline. Chlorides and sulfates will cause corrosion problems at relatively low levels (about 100 PPM). Magnesium and calcium ions will react with emulsifiers to form hard water deposits.

The most reliable method for estimating individual ions is ion chromatography. Heavy metal ions may be determined by X-ray or atomic absorption spectroscopy. There are titration and colorimetric methods for many ions, but these are subject to interference by normal fluid components.

Dirt, in the form of metal fines or grinding wheel grit, is suspended in the fluid and is removed from the fluid by filtration or settling. The level of dirt (total suspended solids) in the fluid is a measure of the efficiency of the filtering system, so the amount of dirt is often checked.

It can be estimated visually by centrifugation of a sample or gravimetrically from a filtered sample.

To test gravimetrically, a 100-mL sample of MRF is filtered through a membrane of known micrometer size (usually 8 micrometers). The dirt captured on the membrane is weighed and reported in parts per million by weight.

A problem with the gravimetric test is that particles less than 8 micrometers pass through the membrane and are never included in the PPM results. In cast iron and cast aluminum, a significant amount of the contaminant may be less than 8 micrometers and will go undetected. Previously, it was thought that small particles below 8 micrometers were not as detrimental to machining operations as larger particles. Recent studies have indicated this is not true; in fact, particles smaller than 8 micrometers may cause more problems than larger particles. Most filter systems for machining operations are effective at removing particles over 20 micrometers. But as the particle size decreases, a filter system’s efficiency in removing particles drops.

With centrifuge testing (see emulsion stability), the total solids reported include many particles in the 1- to 10-micrometer range that the filter cannot efficiently remove. The particulate collected in the bottom of the tube is then reported in parts per million by volume.

Plants are encouraged to use both methods of testing, gravimetric and centrifuge. The centrifuge method should be used regularly to monitor the stability and quality of the MRF. The gravimetric method should be used periodically and when quality problems arise to verify filter function. A gravimetric test through a series of different membranes (1, 8, 20, 40 micrometers, for example) is a better indication of filter performance. The presence of a great number of large particles would indicate a filter or media problem.

Dirt levels can be reduced by use of additives or by adjusting filter parameters.

Microbes

Water-diluted metal removal fluids can support the growth of some microorganisms, as can straight oils contaminated by water. The microorganisms found in metal removal fluids are the same common species that are found in soil, air and on the human body. Because MRF systems can support the growth of microorganisms, it is important to make periodic measurements of their concentration in water-diluted fluids. If doubt exists with miniature and small sumps, DCR (drain, clean, and replace) the fluid instead of running the test.

What’s the procedure for checking for microbial contamination?

Checking for microbial contamination can be done in various ways:

plate counts

dipsticks

dissolved oxygen reading

measurement of catalase levels

Dilution plate counts

Microbial levels may be estimated by preparing increasing dilutions of the MRF in sterile nutrient solution, spreading a known amount of the different dilutions on nutrient agar in Petri dishes, and incubating for two or three days. Each living organism in the sample will multiply to form a visible colony. The number of colonies will be related to the number of living organisms present in a milliliter of the starting fluid and the dilution that produced a countable Petri dish. The results are normally reported as colony-forming units per mL of fluid (CFU/mL). While a general-purpose nutrient medium is used for most testing, the type of microorganism that will grow can be selected by the choice of nutrients used.

Dilution plate counts have a disadvantage. Bacteria increase at an exponential rate. During the 24 to 48 hours that the dipsticks are incubating, bacteria counts can increase by orders of magnitude. If you suspect the significant bacterial or fungal growth, it is better to act than to "wait and see."

Dipsticks

These are available with a general-purpose nutrient agar, or a fungi specific agar, coated on a paddle and enclosed in a clear sterile tube. The paddle is dipped into the sample, returned to its container, and incubated for two days. A dye makes bacterial colonies more visible. Because there is no dilution of the fluid, the number of colonies can be very high. The paddles are read by comparing their appearance to reference pictures that were prepared from known concentrations of bacteria or fungi.

Bacteria increase at an exponential rate. During the 24 to 48 hours that the dipsticks are incubating, bacteria counts can increase by orders of magnitude. If you suspect the significant bacterial or fungal growth, it is better to act than to "wait and see."

Dissolved oxygen

Collecting a fluid sample and immediately taking readings of the dissolved oxygen level can be a quicker method of estimating the microbiological activity in a fluid. Dissolved oxygen probes are readily available.

A reading of the dissolved oxygen level is taken immediately after the fluid sample has been taken. The sample is left undisturbed for two hours, and then a second reading is taken. The rate at which the dissolved oxygen concentration decreases is related to the concentration of bacteria. As a rule of thumb, the second reading should be greater than 6 PPM. If it is lower than 6 PPM, take action immediately to control microbial growth. Otherwise, problems will be noticeable within two days.

Catalase levels

A kit is available that estimates microbial activity from the amount of oxygen evolved from the action of the enzyme catalase on hydrogen peroxide. Some fluid components and contaminants seem to interfere with results, but some users report useful data.

What are acceptable levels of microbes?

Most fluids will tolerate bacterial levels of 10³ organisms /mL with no adverse effects. However, water-diluted fluids can reach levels as high as 10⁸ organisms/mL. There are also some fluids that are "bio-stable" that operate at high bacterial levels without developing odors. There is no consensus on what level of bacterial contamination is safe. It has been suggested that concentrations greater than 10⁵/mL should be controlled.

Fungal levels should be held to less than 100/mL. Fungal cells are normally part of a mass of cells. The presence of even a small number of free-floating cells is an indication of a larger number somewhere in the system that needs attention.

If counts are increasing with time, the fluid should be treated with a biocide before problems are evident. It is much easier to keep counts at a low level by early treatment than it is to bring counts down from a high level. When using a biocide, always treat the system at the recommended level. Use of any biocide at low levels will decrease its effectiveness. The user must read, understand, and follow all appropriate instructions for the handling, storage and use of each biocide, as specified by the biocide manufacturer, and must adhere to all regulatory requirements.

How often should each test be performed?

The chart below summarizes how often the most important fluid tests should be performed, depending on the size of the sump.

Sump Class	Sump Size	MRF Evaluation Method	Interval
Miniature	0.1 to 4 gallons	visual inspection	daily
_________		odor	daily
Small	5 to 20 gallons	visual inspection	daily
_________		odor	daily
_________		concentration (sump level)	daily
Small-Medium	26 to 80 gallons	visual inspection	daily
__________		odor	daily
		concentration (sump level)	daily
		concentration (refractive index)	weekly
		pH	weekly
Medium	100—1000 gallons	visual inspection	daily
___________		odor	daily
		concentration (refractive index)	daily
		pH	daily
Large	over 1000 gallons	visual inspection	daily
__________		odor	daily
		concentration (refractive index)	daily
		concentration (acid) split or other	twice weekly
		pH	daily
		microbiological (dissolved oxygen)	twice weekly
		microbiological (culture)	weekly